γ-Amiobutyric Acid Metabolism in Plants

The metabolism of γ-aminobutyric acid (γAB) by two yeasts, Saccharomyces cerevisiae and Torulopsis utilis, was investigated. Both yeasts grew well upon γAB as a sole source of nitrogen (N), and the lag phase for Torulopsis was shorter than when $NH4+$ provided the N-source. The metabolism of γAB by Torulopsis, which was associated with an increased O₂ uptake, was adaptive in character. The enzyme whose formation was induced by the supply of γAB was a transaminase, which was apparently specific for γAB as the amino donor. Small amounts of transaminase were present in unadapted, $NH4+$-grown cells. The optimum pH, equilibrium constant, Michaelis' constant, and coenzyme requirement were investigated for the transamination reaction involving α-ketoglutaric acid (αKG) as amino group acceptor. Succinic semi-aldehyde (SSA) was a product of this transamination reaction.The possibility; that some γAB was converted into SSA by a direct oxidative deamination remained unconfirmed. The further conversion of SSA into succinic acid was established using intact. cells for both yeasts. This oxidation process was shown to be linked to the reduction of pyridine nucleotides vising extracts of Saccharomyces as a source of SSA dehydrogenase. Dehydrogenase activity could be ascribed to two separate enzymes, one linked to DPN, and the other utilizing TPN and requiring Mg⁺⁺ as an activator. The properties of the former enzyme, which was more important quantitatively, were investigated and compared with those described in the literature for an aldehyde dehydrogenase of baker's yeast and for SSA dehydro-genases of Pseudomonas. Torulopsis extracts could catalyse the reduction of SSA to γ-hydroxybutyric acid (γOHB); the γOHB dehydrogenase involved required TPNH as a coenzyme. Certain other properties of this enzyme are recorded. The possibility is discussed that γAB and SSA act as intermediates in a metabolic pathway that may form a by-pass of the αKG-succinate stage of the tricarboxylic acid cycle.